In the manufacture of a stretchable web such as creped tissue paper, the basis weight of a stretchable web is an important parameter. Basis weight is a measure of mass per unit area of the web. Basis weight is usually expressed in terms of grams per square meter. Typical basis weight values may range from ten to seventy grams per square meter. As will be more fully described, there are prior art systems that exist that measure the basis weight of a stretchable web in real time during the manufacturing process of the stretchable web.
The principles of the present invention will be described with reference to the measurement of a basis weight of creped tissue paper. It is understood that the principles of the invention are not limited to the particular example of creped tissue paper and that the principles of the invention are applicable to the measurement of basis weight for all types of stretchable webs, including, without limitation, all types of creped or embossed tissue material and paper towels.
FIG. 1 illustrates a schematic representation of an exemplary prior art machine 100 for making creped tissue paper. A source (not shown) provides an aqueous slurry of paper fibers to a headbox 110. The headbox 110 deposits the slurry onto a first wire structure 120. The first wire structure allows water from the slurry to drain away and leave a web of paper fibers on the first wire structure 120. The first wire structure 120 that carries web of paper fibers is moved laterally in a continuous loop by a plurality of rollers as shown in FIG. 1.
The web of paper fibers is transferred to a press felt 130 as shown in FIG. 1. The press felt 130 carries the web of paper fibers to a pressure roll 140. The pressure roll 140 transfers the web of paper fibers to surface of a creping cylinder 150. The creping cylinder 150 (also commonly referred to as a Yankee dryer 150). The Yankee dryer 150 dries the web of paper fibers as the Yankee dryer rotates.
The dried web of paper fibers is subsequently removed from the Yankee dryer 150 by the application of a creping doctor 160. The creping doctor 160 comprises a creping blade that forms crepe structures in the web of paper fibers. The resulting creped web of paper fibers is collected on a reel drum 170.
The basis weight of the resulting creped web of paper fibers may be measured in real time using measuring devices (not shown in FIG. 1) that are located within a device that is referred to as a reel scanner 180. The reel scanner 180 is located between the creping doctor 160 and the reel drum 170. The creped web of paper fibers passes through the reel scanner 180. During the continuous manufacture of the creped web of paper fibers, the measuring devices that are located within the reel scanner 180 are employed to measure the basis weight of the creped web of paper fibers at any desired time.
FIG. 2 schematically illustrates three prior art basis weight measuring devices that are used to measure the basis weight of a creped web of paper fibers. The creped web of paper fibers is designated with reference numeral 205. Assume that the web 205 in FIG. 2 is moving laterally from left to right. The three basis weight measuring devices that are shown in FIG. 2 are illustrated for descriptive purposes. In an actual implementation it is likely that only one basis weight measuring device would be used.
The first basis weight measuring device comprises a source 210 and a detector 220 of beta particle radiation. The source 210 exposes the web 205 to beta particles. Some of the beta particles penetrate the web 205 and reach the detector 220 that is located on the other side of the web 205. The beta particle detector 220 measures how many beta particles have penetrated the web 205. By knowing the original intensity of the beta particle radiation from the source 210 and the detected intensity of transmitted beta particle radiation at the detector 220, one can determine an estimate of the basis weight of the web 205 in real time.
The second basis weight measuring device comprises a light source 230 and a light detector 240. The source 230 exposes the web 205 to light having a selected wavelength. A portion of the light that is incident on the web 205 penetrates the web 205 and reaches the detector 240 that is located on the other side of the web 205. The light detector 240 measures how much light penetrates the web 205. By knowing the original intensity of the light from the light source 230 and the detected intensity of the transmitted light at the light detector 240, one can determine a rough estimate of the basis weight of the web 205 in real time.
The third basis weight measuring device comprises an infrared source 250 and an infrared detector 260. The source 250 exposes the web 205 to infrared light having at least two selected wavelengths. A portion of the light that is incident on the web 205 is reflected from the web 205 and reaches the infrared detector 260 that is located on the same side of the web 205. The infrared detector 260 measures the ratio of wavelengths reflected from the web 205. By knowing the ratio, one can determine an estimate of the basis weight of the web 205 in real time.
The estimate of the basis weight of the web 205 can be used as feedback information to control the manufacturing process of the web 205. For example, basis weight values can be used to control a fan pump that regulates the amount of slurry material that is provided to the headbox 110. Basis weight values can also be used as an indicator of blade wear of the creping blade in the creping doctor 160. It is therefore important to obtain a measurement of the basis weight of the web 205 that is as accurate as possible.
The velocity of the web 205 goes to zero as the web 205 encounters the creping blade of the creping doctor 160. The web 205 then accelerates back to machine velocity on its way to the reel drum 170. Due to the creping of the web 205, the web 205 is somewhat elastic. Therefore the velocity of the web 205 oscillates around the value of the machine velocity as the web 205 moves from the creping doctor 160 to the reel drum 170.
To accommodate the various velocities, the crepes are either pulled out or compressed. Depending upon the location where the basis weight measurement is made, there could be more material or less material under the sensor of the basis weight measuring device than there would be in the finished web 205 at rest. Furthermore, the rate at which the crepe is pulled out between the creping doctor 160 and the reel drum 170 may be different depending upon factors such as the condition of the creping doctor 160, the weight of the web 205, moisture content, etc. Variations in these factors may cause the basis weight measurement of the web 205 to be in error.
To compensate for these variations some prior art approaches measure the velocity of the web 205 at the location where the basis weight measurement is made and then compare the measured velocity with the velocity of reel drum 170. Then a correction is calculated to obtain a more accurate value for the basis weight measurement.
It would be desirable to have an even more accurate and precise method for correcting a basis weight measurement of a stretchable web in real time during the manufacturing process of the stretchable web.